1. Field of Invention
This invention relates to digital communications (either traditional wireless communications or the reading of digitally stored information from an optical or magnetic media) for increasing the throughput of transmitted bits in a multiuser detection (MUD) based multiple access communication system in which multiple users transmit information simultaneously on the same channel or frequency, causing purposeful interference, by employing multiuser detection algorithms in each receiver where each receiver also employs multiple collectors.
2. Description of Related Art
Multiple access communication systems allow the transmission of multiple digital data streams between multiple transmitting and receiving devices. Cellular or PCS systems provide high quality voice service in a wide-ranging geographical coverage area at relatively low cost. However, since many users transmit energy on the same communications channel, a number of inherent difficulties arise, particularly when a large number of user receivers attempt to detect the information associated with a particular user when there is heavy signal interference created by other users of the system at the same time. Typically the signal of interest cannot be received or the quality of reception is significantly degraded.
One way of solving this problem is to separate the interfering transmissions at the receiver, using signal processing techniques. However, today's current state of the art receivers are not capable of detecting and decoding the information associated with each user under conditions of heavy interference. Another solution to the co-channel interference problem is to decrease the number of users per channel. This, of course, is not an attractive option for telecommunication companies, since obtaining the maximum number of users or managing peak volume transmission periods are important business objectives.
Several techniques exist to improve results in co-channel multiple access communications systems. Frequency-Division Multiple Access (FDMA) assigns a different frequency to each user. In a cellular telephone configuration this poses problems because all proximate cells must operate on different frequencies. However, frequency bands may be re-used, provided that the same frequency cells are positioned at a certain distance apart. A further drawback with FDMA schemes is that users will pay full-time for their assigned frequency regardless of their actual use of the system. Additionally, the number of users will be restricted to the number of dedicated frequencies that can be provided and one modem is required for each user therefore leading to high hardware and operational costs.
In Time Division Multiple Access (TDMA) technology, multiple channels of data are temporally interleaved, i.e. each signal is assigned to a different time interval and the signals are transmitted individually, according to their assigned time slot. However, in a TDMA system, all transmitters and receivers must have access to a common clock, as time-synchronization among the users is required. Also, as for FDMA, there is only a limited number of time slots available for assignment which can cause a system to be fully loaded even when there are many tens of users requesting a channel.
Code Division Multiple Access (CDMA) is another multiplexing technique wherein for each communication channel, the signals are encoded using a sequence known to the transmitter and the receiver for that specific channel. In CDMA, all users use the same frequency at the same time. However, before transmission, the signal from each user is multiplied by a distinct signature waveform. The signature waveform is a signal that has a larger bandwidth than the information-bearing signal from the user. However, in a CDMA system, the total level of co-channel interference limits the number of active users at any instant of time. The present invention involves improvements made to techniques of multi-access system reception to solve practical problems associated with increasing the number of simultaneous users for any actual commercial system that employs algorithm signaling such as TDMA, FDMA or a hybrid of the two.
It is possible to increase overall throughput in terms of total number of bits transmitted through the shared medium, by all users of a multiple access system if the signals are allowed to interfere. Current state of the art receivers, however, are not capable of detecting and decoding the information associated with each user when there is heavy interference.
Multiuser detection systems take full advantage of all knowledge available at the receiver, by making use of any knowledge that the receiver has about the interference signals. The ability of multiuser detection procedures to perform the required processing in real time to produce reliable symbol estimates is a known drawback of multiuser detection based systems. This is due to the fact that as the total number of interfering signals goes up, the non-optimal MUD algorithms that perform short cuts in detection and estimation of symbols relative to the maximum likelihood exhaustive search MUD algorithm begin to fail. Moreover, as the number of users is increased beyond the number of dimensions (independent receiver measurements) the algorithm mathematics become ill defined and cannot be computed.
Therefore a need exists to overcome the problems with the prior art as discussed above, and particularly for a system and method for increasing the throughput of transmitted bits in a multiuser detection (MUD) based multiple access communication systems in which multiple users transmit information simultaneously on the same channel or frequency by employing multiple collectors at the receiver.